In a typical communication network, also referred to as a communication system, wireless communication system or wireless communication network, user equipments communicate via a Radio Access Network (RAN) to one or more core networks (CNs).
A user equipment is a device by which a subscriber may access services offered by an operator's core network and services outside operator's network to which the operator's radio access network and core network provide access, e.g. access to the Internet. The user equipment may be any device, mobile or stationary, enabled to communicate over a radio channel in the communications network, for instance but not limited to e.g. mobile phone, smart phone, sensors, meters, vehicles, household appliances, medical appliances, media players, cameras, or any type of consumer electronic, for instance but not limited to television, radio, lighting arrangements, tablet computer, laptop or Personal Computer (PC). The user equipment may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment or a server.
User equipments are enabled to communicate wirelessly with the communications network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks and possibly the Internet.
The communications network covers a geographical area which may be divided into cell areas, and may therefore also be referred to as a cellular network. Each cell area is served by a Base Station (BS), e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. NodeB, B node, or Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations communicate over the air interface with the user equipments within range of the base stations. The base station is referred to as BS in some of the drawings.
In the user equipment and/or the base station, a received radio signal is converted from the analogue domain to the digital domain before further baseband signal processing is performed. In most digital signal processing systems, upon reception of a digital signal, the first stage of processing is to compress the received signal by means of a Digital Automatic Gain Control (DAGC). The reason for compressing the incoming signal is to decrease the amount of processing cycles and memory needed (in the baseband domain), and thus in the end reduce the hardware cost and power consumption.
If signals input to the DAGC changes faster than the DAGC is designed for, the compressed output signal will be distorted and information will be lost during the processing.
In a (Wideband) Code Division Multiple Access system, abbreviated (W)CDMA, W-CDMA or WCDMA, a distorted signal will result in a degraded performance of the received signal quality.
According to the Third Generation Partnership Project (3GPP), “W-CDMA is a spread-spectrum modulation technique; one which uses channels whose bandwidth is much greater than that of the data to be transferred. Instead of each connection being granted a dedicated frequency band just wide enough to accommodate its envisaged maximum data rate, W-CDMA channels share a much larger band. The modulation technique encodes each channel in such a way that a decoder, knowing the code, may pick out the wanted signal from other signals using the same band, which simply appear as so much noise”.
A key feature in a (W)CDMA system is fast transmit power control often referred to as Inner Loop Power Control (ILPC). The ILPC is a feedback loop between the user equipment and the base station aiming to keep the received signal to noise level, i.e. Signal to Interference Ratio (SIR), at a constant level. This is done by one or more TPC commands, which indicates a reduction or an increase of the power. The ILPC may be in the downlink or in the uplink. In the uplink, ILPC refers to the ability of the user equipment to adjust its output power in accordance with one or more TPC commands received in the downlink, in order to keep the received uplink SIR at a given SIR target. In the downlink, the ILPC refers to the ability of the base station to adjust its output power in accordance with one or more TPC commands received in the uplink. Uplink may be defined as a transmission from the user equipment to the base station and downlink may be defined as a transmission from the base station to the user equipment.
SIR is commonly used as a way to measure the quality of wireless connections. SIR is calculated as SIR=P/(I+N) where P is signal power, I is interference power and N is noise power.
When the input signal to a DAGC is fairly uniform the amount of signal distortion due to the DAGC may be low. But as the uniform distribution is altered, the amount of distortion is increased.
TPC is an abbreviation for Transmit Power Control or Transmit Power Commands and is used in order to prevent too much unwanted interference between network cells and user radio links in a WCDMA radio access network. Cell power is a shared resource in WCDMA and abundant power is undesired. In general for any radio access network, the power control enables reduced energy consumption. The UL TPC mechanism dynamically adjusts the UL transmission power. For each radio link, the uplink inner loop power control continuously adjusts the transmit power of the user equipment in order to keep the received uplink SIR substantially at a given SIR target. Each radio link comprises an UL and a DL. The UL carries TPC commands to control DL power (this is called DL TPC in this document). The DL carries TPC commands to control UL power (this is called UL TPC in this document).
Efficient power control is crucial in a (W)CDMA communications network in order to minimize interference between radio channels in the network. Power control ensures that each user equipment receives and transmits just enough energy to properly convey information while interfering with other user equipment's no more than necessary.
Due to increase of packet switched data in (W)CDMA systems today, the input power of the received signal may rapidly change and the DAGC may suffer of more distortion.
The received signal comprises both user data and control data. When the received signal is distorted, both the user data and the control data will also be distorted. Distortion of user data normally only leads to degraded performance for that specific user equipment, but distortion of control data may lead to errors in the ILPC feedback loop. Errors in the ILPC feedback loop affect not only the specific user equipment but all user equipments in the system as the SIR cannot be kept at the desired level.
When a user equipment has a SIR larger than a target value the ILPC should send a TPC command indicating power down. The SIR of the user equipment is measured as received power divided by noise power. When the received signal is distorted due to errors in the DAGC, the estimation of the received power may be too low, and thus the estimated SIR is also too low. If the error is large enough, the estimated SIR may be lower than the target value instead of larger than the target value. In this case the resulting TPC command will indicate power up instead of power down.
As increased power may lead to even higher distortion the problems may continue with the result that the transmitting equipment ends up transmitting at its maximum power. This phenomenon is often referred to as a power rush.
Power rushes are severe in the communications network since they may lead to increased interference or noise for other user equipments, and the accessibility and retainability of the communications network is decreased.
Upcoming WCDMA features are expected to increases the probability and amount of distortion and errors.